A liquid crystal display (LCD) device comprises a pair of substrates having transparent electrodes or switching devices, arranged to face each other with a space in which a liquid crystal material is filled. Spacers are typically placed between the two substrates in order to maintain a gap and control the thickness formed by the substrates. FIG. 1 shows a conventional LCD with spacers between the substrates. The conventional LCD structure comprises an upper polarizer 11, a middle structure including liquid crystal cells 12 and spacers 13, a lower polarizer 14, and a light is source 15.
It is important that an LCD device has a uniform cell gap to achieve high contrast, wide viewing angle and low color distortion. In general, For a twisted nematic (TN) LCD, the accuracy of the uniform thickness of the cell gap has to be controlled within .+-.0.1 .mu.m. For a super twisted nematic (STN) LCD, it requires that the spacing be controlled within .+-.0.05 .mu.m for high quality. A conventional method of manufacturing spacers in a thin-film transistor (TFT) LCD is illustrated in FIG. 2. Spacer balls are uniformly sprayed and distributed on top of the substrate as shown in FIG. 2.
The main drawback in the LCDs manufactured by the conventional approach is that the spacers may block some of the pixel area. The pixel aperture ratio is thus decreased due to the spacers. Both the brightness and the contrast of the display are degraded. The problem becomes very severe for LCDs having a smaller pixel size. As an example, for projection type or high definition LCDs, the pixel size can be as small as 40 .mu.ms. If a spacer ball of a 5 um size falls in a pixel area, the quality of the displayed picture is greatly impacted.
Techniques of maintaining the spacing between the LCD substrates have been presented to overcome the problems associated with spacer balls. U.S. Pat. No. 5,268,782 of Wetiz et al. discloses a liquid crystal display device using micro-ridged polymeric substrates for maintaining a precise and uniform spacing between substrates. H. Yamanaka et al. presented a paper titled "Integrated Black Matrix on TFT Arrays" in SID 92 DIGEST for manufacturing an integrated black matrix that includes pigment-dispersed photo-polymer on TFT array in order to improve aperture ratio. H. Yamashita et al. published a paper titled "Precise Cell-Thickness Control by Spacer-Ball-Free Structure and its Application to Large-Size TFT LCDs" in SID 96 DIGEST for controlling the cell thickness by a stack of spacer-ball-free hybrid column structure that combines organic black matrix and color filter layers.
FIG. 3 shows an LCD device having photo-polymer layers to control the thickness of the cell gap. It can be seen that the pixel area is not blocked by any spacer ball. This type of technique, however, has a drawback in that it is difficult to fabricate a photo-polymer layer with substantial thickness such as 5.about.6 .mu.ms required in an LCD device. Two photo-polymer layers are often required to form a column stack structure. The structure may also be manufactured by fabricating a photo-polymer layer on each of the top and bottom substrates. The two photo-polymer layers are then aligned to control the thickness of the cell gap. The process and cost of manufacturing LCDs are increased due to the formation of two photo-polymer layers or the difficulty in the alignment. A better method for controlling the cell thickness in the fabrication of LCDs is highly desirable in the industry.